Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Thu Mar 07 2024 Valid 12Z Sun Mar 10 2024 - 12Z Thu Mar 14 2024 ...Strong storm to bring rain/snow and high winds to the Northeast Sunday into the next work week... ...Multi-day unsettled period along the northern-central West Coast continuing into early next week... ...Overview... Deepening and consolidating low pressure will bring a period of rain/snow and high winds to the Northeast during Sunday-Monday while a broad area of dry weather prevails behind this system as far west as the central-southern Rockies and Southwest. The series of shortwaves and surface fronts bringing rain and mountain snow to the Pacific Northwest/northern California will already be in progress at the start of the period early Sunday and should continue into Tuesday. These systems should extend snow into the northern Rockies. The final upper trough in this series should then move inland and amplify by Wednesday-Thursday as an upper ridge amplifies/sharpens off the West Coast. The trough may close off an upper low over the Four Corners or Southwest. This feature will push the West Coast moisture into the central Rockies by Wednesday and ultimately interact with increasing low level Gulf of Mexico inflow to start generating rainfall over the southern Plains by around next Thursday. A mean ridge aloft setting up over the central/east-central U.S. will support an expanding area of above normal temperatures to the east of the Rockies, with the greatest anomalies likely to be over the northern Plains/Upper Midwest early-mid week. ...Guidance/Predictability Assessment... Latest guidance continues to offer steady convergence for the system affecting the Northeast early in the week. Dynamical models/ensembles and machine learning models all agree fairly well upon an upper low tracking from the Great Lakes to just off the New England coast early Sunday into early Monday, while among two initial surface lows (northern of Lake Ontario and far southern New England) the southern one becomes the dominant system and reaches a position near Downeast Maine by early Monday. Over recent runs the UKMET has been the most reluctant to trend toward consensus (tending to be slow), and while the new 00Z run has finally adjusted to consensus aloft its surface low is west of the majority as of early Monday. This system should track eastward thereafter. A model blend emphasizing the GFS/ECMWF provides a reasonable starting point for this system. Guidance continues to show typical variability/spread for the shortwaves coming into the West, and especially for the one that arrives by early Monday and continues across the lower 48 thereafter. Solutions offer a general signal for general area of low pressure/frontal system reaching the central U.S. by Tuesday, but surface depictions increasingly diverge by Wednesday-Thursday due to low-predictability differences in details aloft as the energy passes through the longer-term mean ridge. As the final upper trough moves into the West by Wednesday-Thursday, GFS/GEFS trends over the past day or so have led to improved agreement toward an amplified scenario advertised by earlier ECMWF/CMC runs and their means. Most of the recent runs from ECMWF-initialized machine learning models offer reasonable support for the current dynamical model/ensemble mean consensus for the late-period amplifying upper trough. Guidance comparisons for the 12Z/18Z cycle led to starting the updated forecast with an operational model composite emphasizing more GFS/ECMWF weight for the first half of the period, followed by a transition toward 30-40 percent total 18Z GEFS/12Z ECens inclusion along with the GFS/ECMWF/CMC by Wednesday-Thursday as some details become more ambiguous. ...Weather/Hazards Highlights... During the Day 4 Excessive Rainfall Outlook period covering Sunday-Sunday night, most guidance shows the southern of two separate surface low centers as of early Sunday (over southern New England, versus the other one north of Lake Ontario) becoming the main low along the northern New England Coast and reaching near Downeast Maine by early Monday. During the day Sunday there will be a period of strong Atlantic inflow ahead of the advancing low, with recent guidance fairly consistent in showing significant QPF centered over Maine on top of wet ground from shorter term rainfall. Thus the Day 4 ERO maintains a Marginal Risk area over the southern half or so of Maine, with a modest trimming of the northern area to account for average guidance location of the rain-snow line. Winter Weather Outlook probabilities continue to show greater than 50 percent chances for at least 0.25 inch liquid in the form or snow over portions of interior New England on Sunday. Some lake effect/terrain-enhanced snow should extend farther westward. Lighter snow may continue over northern New England into Monday. Meanwhile this storm may produce high winds during Sunday-Monday over some areas from the central Appalachians/Mid-Atlantic into the Northeast. The rapid succession of frontal systems bringing multiple days of rain and mountain snow to the Pacific Northwest and northern California should already be in progress at the start of the period early Sunday and will likely continue into Tuesday before an approaching ridge supports a drier trend. Some of this moisture will bring snow into the northern Rockies. The Days 4-5 EROs depict a Marginal Risk area along the Pacific Northwest into northern California coast for this event. Guidance continues to show some mixed messages for details of magnitude/distribution of moisture and rainfall totals, but wet ground conditions should lower the threshold for localized runoff issues where the relatively higher rainfall rates occur. As the final upper trough in the series amplifies and moves inland, expect the best focus for rain and mountain snow to shift into the central Rockies by Wednesday. Then rainfall may increase in coverage and intensity over the southern Plains by next Thursday. Mean ridging aloft over the central/east-central U.S. will support an expanding area of above normal temperatures to the east of the Rockies. The core of warmest anomalies should cover parts of the northern Plains into the Upper Midwest during Monday-Wednesday, with highs potentially at least 20-25F above normal. Lesser warm anomalies will gradually displace the near or slightly below readings over the South and East as of Sunday-Monday. Over the West, the wet/snowy pattern over northern-central West Coast and eventually into the Rockies will maintain moderately below average highs and near/above average lows through early next week. Upper troughing that amplifies inland by Wednesday-Thursday should spread a broader area of cool daytime highs across the West with lows declining toward normal. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw